Skype for Business: Cannot share screen or receive content

Hi,

Using Skype for Business (latest Version .87), trying to share my screen fails. Clicking on the share button, a thick yellow line appears around the Desktop, it says "looking for Meeting Content" on top, the line then thins out and disappears, nothing is shared.

Users at the other side (Windows 10) see "an error has occurred during the presentation".

Also, I can't see Content that others try to share.

Running Mojave on an iMac 5k, Office 365 and Skype for Business in their regarding latest versions with all updates applied.

Tried to:

- Erase all files and Caches of SfB, Lync "residuals" (didn't find any) and Office 365, reinstalling everything

- Trying to Play with Audio & Video settings

- involving Microsoft -- they say the can't reproduce the error in their labs -- following all their tips to remove skype, reinstall, update everything

- Even "uninstalling" Office 365 and Skype following MS instructions and Setting up a fresh user on the iMac didn't help

Important Notes: Screen sharing has actually worked *once* on the iMac before; not sure whether this was shortly before my update to Mojave; however MS says that SfB is now fully compatible with Mojave, and tested.

Video, Audio, Chat works fine in SfB on my iMac. Just Screen sharing, which is a "Business Show stopper" for using the iMac in my work Environment, Fails.

Thanks for any help - even Microsoft seems to be stuck and I can't use my iMac for work

Also, IMPORTANT SfB works and has always worked on my MacBook 12" Retina (early 2016), and there I have installed Skype for Business after the update from Sierra to Mojave. Compared Setup including Network Settings to my iMac, and I couldn't find any differences. However that indicates it isn't impossible ;-)

Any help is greatly appreciated!

Martin